In recent times, wiring density and mounting density in substrates such as printed circuit boards, wafers, flexible substrates and the like (which hereinafter may also be referred to as “mounting substrates”) have become increasingly improved. A mounting substrate has a number of electrodes for soldering electrical components thereto. On such electrodes, solder for soldering electrical components, such as solder bumps, solder paste or the like (hereinafter, such solder may also be referred to as “connecting solder”), is provided. Thereafter, the electrical components are soldered to the connecting solder and mounted onto the mounting substrate.
Connecting solder needs to be microscopic and uniform in terms of the shape and dimensions, etc. thereof, and also to be provided only at required parts. As a method for forming connecting solder that satisfies such requirements, Patent Document 1 proposes an approach, or the like, in which paste bumps having a precise and constant shape are easily formed by making use of a screen plate which is provided with openings for forming paste bumps with paste and which is characterized by being consisting of a rigid first metal layer, a resin-based adhesive layer and a second metal layer, wherein the diameter of the openings in the adhesive layer and the second metal layer is reduced with respect to that of the openings in the first metal layer.
Incidentally, variations may be present in the dimensions of connecting terminals, such as lead terminals, etc., of electrical components such as a connector, a QFP (Quad Flat Package), an SOP (Small Outline Package), a BGA (Ball Grid Array), an LGA (Land Grid Array), or the like. In order to solder electrical components having connecting terminals with varied dimensions without causing defective soldering, it is necessary to reduce the influence due to the variations in dimensions of the electrical components by increasing the thickness of the “connecting solder” to be provided on a mounting substrate. When small-scale electrical components, such as a CSP (Chip Size Package) or the like, are mixed in the electrical components for mounting on to the mounting substrate, the size of the connecting solder for such small-scale electrical components is very small and microscopic.
As a general method of forming connecting solder, it is known to dip (immerse) a mounting substrate provided with electrodes made of copper or the like (for example, copper electrodes; the same applies hereinafter), as is, into molten solder. When solder makes contact with the copper electrode, a CuSn intermetallic compound is produced by copper and tin contained in the solder combining together. This production of a CuSn intermetallic compound itself is the basis of a solder joint. However, such phenomenon is sometimes also referred to as “electrode erosion,” as it occurs in such a manner that the copper electrodes are eroded by the tin contained in the solder. Such electrode erosion decreases reliability by reducing the volume of the copper electrodes for connecting the electrical components thereto and thus, it is likely that the reliability of the mounting substrate will be deteriorated. Consequently, it is necessary to suppress such electrode erosion by shortening the time during which the mounting substrate is dipped into molten solder. Therefore, a method (dipping method) has been discussed in which a preliminary solder layer is formed over the copper electrodes of the mounting substrate and then, the mounting substrate is dipped into molten solder.